Jupiter is the fifth planet from the Sun and is the largest planet in
the solar system. If Jupiter were hollow, more than one thousand Earths
could fit inside. It also contains two and a half times the mass of all the other
planets combined. It has a mass of 1.9 x 1027 kg and is
142,800 kilometers (88,736 miles) across the equator. Jupiter possesses
62 known satellites. The four largest are Callisto,
Europa, Ganymede and
Io, and were named after
Galileo Galilei who observed them as long ago as 1610.
The German astronomer Simon Marius claimed to have seen the moons around the same
time, but he did not publish his observations and so Galileo is given the credit
for their discovery.

Jupiter has a very faint ring system, but is totally invisible
from the Earth. (The rings were discovered in 1979 by Voyager 1.)
The atmosphere is very deep, perhaps comprising the whole planet, and is
somewhat like the Sun. It is composed mainly of hydrogen and helium,
with small amounts of methane, ammonia, water vapor and other compounds.
At great depths within Jupiter, the pressure is so great that
the hydrogen atoms are broken up and the electrons are freed so that
the resulting atoms consist of bare protons.
This produces a state in which the hydrogen becomes metallic.

Colorful latitudinal bands, atmospheric clouds and storms illustrate
Jupiter's dynamic weather systems. The cloud patterns change within
hours or days. The Great Red Spot is a complex
storm moving in a counter-clockwise direction. At the outer edge,
material appears to rotate in four to six days; near the center, motions
are small and nearly random in direction. An array of other smaller
storms and eddies can be found through out the banded clouds.

Auroral emissions, similar
to Earth's northern lights, were
observed in the polar regions of Jupiter. The auroral emissions appear
to be related to material from Io that spirals
along magnetic field lines to fall into Jupiter's atmosphere.
Cloud-top lightning bolts, similar to superbolts in Earth's high
atmosphere, were also observed.

Jupiter's Ring

Unlike Saturn's intricate and complex ring patterns, Jupiter has a simple
ring system that is composed of an inner halo, a main ring and a Gossamer
ring. To the Voyager spacecraft, the Gossamer ring appeared to be a single
ring, but Galileo imagery provided the unexpected discovery that Gossamer
is really two rings. One ring is embedded within the other.
The rings are very tenuous and are composed of dust particles
kicked up as interplanetary meteoroids smash into Jupiter's four small
inner moons Metis, Adrastea,
Thebe, and Amalthea.
Many of the particles are microscopic in size.

The innermost halo ring is toroidal in shape and extends radially from
about 92,000 kilometers (57,000 miles) to about 122,500 kilometers
(76,000 miles) from Jupiter's center. It is formed as fine particles of dust from the
main ring's inner boundary 'bloom' outward as they fall toward the planet.
The main and brightest ring extends from the halo boundary out to
about 128,940 kilometers (80,000 miles) or just inside the orbit of
Adrastea. Close to the orbit of Metis, the main ring's brightness
decreases.

The two faint Gossamer rings are fairly uniform in nature. The
innermost Amalthea Gossamer ring extends from the orbit of Adrastea
out to the orbit of Amalthea at 181,000 kilometers (112,000 miles)
from Jupiter's center. The fainter Thebe Gossamer ring extends from
Amalthea's orbit out to about Thebe's orbit at 221,000 kilometers
(136,000 miles).

Jupiter's rings and moons exist within an intense radiation belt of
electrons and ions trapped in the planet's magnetic field. These
particles and fields comprise the jovian
magnetosphere or magnetic
environment, which extends 3 to 7 million kilometers (1.9 to 4.3
million miles) toward the Sun, and stretches in a windsock
shape at least as far as Saturn's orbit - a distance of 750
million kilometers (466 million miles).

Jupiter
This image was taken by NASA's Hubble Space Telescope on February 13,
1995. The image provides a detailed look at a unique cluster of
three white oval-shaped storms that lie southwest (below and to the
left) of Jupiter's Great Red Spot. The appearance of the clouds, in
this image, is considerably different from their appearance only
seven months earlier. These features are moving closer
together as the Great Red Spot is carried westward by the prevailing
winds while the white ovals are swept eastward.

The outer two of the white storms formed in the late 1930s. In the
centers of these cloud systems the air is rising, carrying fresh
ammonia gas upward. New, white ice crystals form when the upwelling
gas freezes as it reaches the chilly cloud top level where temperatures
are -130°C (-200°F).
The intervening white storm center, the ropy structure to the left of
the ovals, and the small brown spot have formed in low pressure cells.
The white clouds sit above locations where gas is descending to lower,
warmer regions.

This true color mosaic of Jupiter was constructed from images taken by the
Cassini spacecraft on December 29, 2000, during its closest approach to
the giant planet at a distance of approximately 10 million kilometers
(6.2 million miles).

It is the most detailed global color portrait of Jupiter ever produced;
the smallest visible features are approximately 60 kilometers (37 miles)
across. The mosaic is composed of 27 images: nine images were required
to cover the entire planet in a tic-tac-toe pattern, and each of those
locations was imaged in red, green, and blue to provide true color.
Although Cassini's camera can see more colors than humans can, Jupiter's
colors in this new view look very close to the way the human eye would see them.

Everything visible on the planet is a cloud. The parallel reddish-brown
and white bands, the white ovals, and the large Great Red Spot persist
over many years despite the intense turbulence visible in the atmosphere.
The most energetic features are the small, bright clouds to the left of
the Great Red Spot and in similar locations in the northern half of the
planet. These clouds grow and disappear over a few days and generate
lightning. Streaks form as clouds are sheared apart by Jupiter's intense
jet streams that run parallel to the colored bands. The prominent dark band
in the northern half of the planet is the location of Jupiter's fastest
jet stream, with eastward winds of 480 kilometers (300 miles) per hour.
Jupiter's diameter is eleven times that of Earth, so the smallest storms
on this mosaic are comparable in size to the largest hurricanes on Earth.

Unlike Earth, where only water condenses to form clouds, Jupiter's clouds
are made of ammonia, hydrogen sulfide, and water. The updrafts and
downdrafts bring different mixtures of these substances up from below,
leading to clouds at different heights. The brown and orange colors may
be due to trace chemicals dredged up from deeper levels of the atmosphere,
or they may be byproducts of chemical reactions driven by ultraviolet light
from the Sun. Bluish areas, such as the small features just north and south
of the equator, are areas of reduced cloud cover, where one can see deeper.
(Courtesy NASA/JPL/Space Science Institute)

The Interior of Jupiter
This picture illustrates the internal structure of Jupiter. The
outer layer is primarily composed of molecular hydrogen. At
greater depths the hydrogen starts resembling a liquid.
At 10,000 kilometers
below Jupiter's cloud top liquid hydrogen reaches a pressure
of 1,000,000 bar with a temperature of 6,000° K. At this
state hydrogen changes into a phase of liquid metallic
hydrogen. In this state, the hydrogen atoms break down yeilding
ionized protons and electrons similar to the Sun's interior.
Below this is a layer dominated by ice where "ice" denotes a soupy
liquid mixture of water, methane, and ammonia under high temperatures
and pressures. Finally at the center is a rocky or rocky-ice core
of up to 10 Earth masses.
(Copyright Calvin J. Hamilton)

Thin Crescent Image of Jupiter
This thin crescent picture of Jupiter was created from a
photomosaic of images Galileo took on its C9 orbit. It is made
from Near Infrared and Violet images, with an artificial green
image produced from the other two.
(Courtesy of Ted Stryk)

Nordic Optical Telescope
This image of Jupiter was taken with the 2.6 meter
Nordic Optical
Telescope, located at La Palma, Canary Islands. It is a good
example of the best imagery that can be obtained from earth based
telescopes.
(c) Nordic Optical Telescope Scientific Association (NOTSA).

Jupiter with Satellites Io and EuropaVoyager 1 took this photo of Jupiter
and two of its satellites (Io, left, and
Europa, right) on Feb. 13, 1979. In this view,
Io is about 350,000 kilometers (220,000 miles) above Jupiter's Great
Red Spot, while Europa is about 600,000 kilometers (373,000 miles) above
Jupiter's clouds. Jupiter is about 20 million kilometers (12.4 million
miles) from the spacecraft at the time of this photo.
There is evidence of circular motion in Jupiter's atmosphere. While the
dominant large scale motions are west-to-east, small scale
movement includes eddy like circulation within and between the
bands. (Courtesy NASA/JPL)

Satellite Footprints Seen in Jupiter Aurora
In this Hubble Space Telescope picture, a curtain of
glowing gas is wrapped around Jupiter's north pole like
a lasso. This curtain of light, called an aurora, is
produced when high-energy electrons race along the
planet's magnetic field and into the upper atmosphere where they excite
atmospheric gases, causing them to glow. The aurora resembles the same
phenomenon that crowns Earth's polar regions. But this Hubble image,
taken in ultraviolet light, also shows the glowing "footprints" of three of
Jupiter's largest moons: Io, Ganymede, and Europa.

Courtesy of NASA/ESA, John Clarke (University of Michigan)

Jupiter's Magnetosphere
This image taken by the ion and neutral mass spectrometer
instrument on NASA's Cassini spacecraft makes the huge magnetosphere surrounding
Jupiter
visible in a way no instrument on any previous spacecraft has been able to do. The
magnetosphere is a bubble of charged particles trapped within the magnetic environment of
the planet.
A magnetic field is sketched over the image to place the energetic neutral
atom emissions in perspective. This sketch extends in the horizontal plane to a width 30
times the radius of Jupiter. Also shown for scale and location are the disk of Jupiter (black
circle) and the approximate position (yellow circles) of the doughnut-shaped torus created
from material spewed out by volcanoes on Io.

Some of the fast-moving ions within the magnetosphere pick up electrons to become neutral
atoms, and once they become neutral, they can escape Jupiter's magnetic field, flying out
from the magnetosphere at speeds of thousands of kilometers, or miles, per second.

Jupiter's Auroras
These HST images, reveal changes
in Jupiter's auroral emissions and how small auroral spots just
outside the emission rings are linked to the planet's volcanic
moon, Io. The top panel pinpoints the
effects of emissions from Io. The image on
the left, shows how Io and Jupiter are
linked by an invisible electrical current of charged particles
called a flux tube. The particles, ejected from Io
by volcanic eruptions, flow along
Jupiter's magnetic field lines, which thread through Io, to the
planet's north and south magnetic poles.

The top-right image shows Jupiter's auroral emissions
at the north and south poles. Just outside these emissions are
the auroral spots called "footprints." The spots are created
when the particles in Io's "flux tube" reach Jupiter's upper
atmosphere and interact with hydrogen gas, making it fluoresce.

The two ultraviolet images at the bottom of the picture show how
the auroral emissions change in brightness and structure as
Jupiter rotates. These false-color images also reveal how the
magnetic field is offset from Jupiter's spin axis by 10 to 15
degrees. In the right image, the north auroral emission is rising
over the left limb; the south auroral oval is beginning to set.
The image on the left, obtained on a different date, shows a
full view of the north aurora, with a strong emission inside the
main auroral oval.

The Great Red Spot
This dramatic view of Jupiter's Great Red Spot and its
surroundings was obtained by Voyager 1 on Feb. 25, 1979, when the
spacecraft was 9.2 million kilometers (5.7 million miles) from
Jupiter. Cloud details as small as 160 kilometers (100 miles)
across can be seen here. The colorful, wavy cloud pattern to the
left of the Red Spot is a region of extraordinarily complex and
variable wave motion. (Courtesy NASA)

False Color of Jupiter's Great Red Spot
This image is a false color representation of Jupiter's Great Red Spot
taken with Galileo's imaging system through three different near-infrared
filters. This is a mosaic of eighteen images (6 in each filter) that were
taken over a period of 6 minutes on June 26, 1996. The Great Red Spot
appears pink and the surrounding region blue because of the particular
color coding used in this representation. The red channel is the
reflectance of Jupiter at a wavelength where methane strongly absorbs
(889nm). Because of this absorption, only high clouds can reflect sunlight
in this wavelength. The green channel is the reflectance in a wavelength
where methane absorbs, but less strongly (727nm). Lower clouds can reflect
sunlight in this wavelength. Finally, the blue channel is the reflectance
in a wavelength where there are essentially no absorbers in the Jovian
atmosphere (756nm) and one sees light reflected from the deepest clouds.
Thus, the color of a cloud in this image indicates its height, with red or
white being highest and blue or black being lowest. This image shows the
Great Red Spot to be relatively high, as are some smaller clouds to the
northeast and northwest that are surprisingly like towering thunderstorms
found on earth. The deepest clouds are in the collar surrounding the Great
Red Spot, and also just to the northwest of the high (bright) cloud in the
northwest corner of the image. Preliminary modelling shows these cloud
heights to range about 50km in altitude.
(Courtesy NASA/JPL)

Ring of Jupiter
The ring of Jupiter was discovered by Voyager 1 in March of 1979.
This image was taken by Voyager 2 and has been pseudo colored. The
Jovian ring is about 6,500 kilometers (4,000 miles) wide and probably
less than 10 kilometers (6.2 miles) thick.
(Copyright Calvin J. Hamilton)

The Jovian System
The best of the Jupiter system is pictured in this collage
of images acquired by the Voyager and Galileo spacecraft.
Jupiter is the largest planet in our solar system. The
four largest moons of Jupiter are known as
the Galilean moons and are named Callisto,
Ganymede, Europa,
and Io. Inside the orbits of the Galilean moons
are Thebe, Amalthea,
Adrastea, and Metis.
At the lower right is shown the Valhalla region of Callisto.
Ganymede is toward the bottom middle. Europa is a little above
and to the right of Ganymede. Io is the top, left-most moon.
Between Io and Jupiter are four little moons. The top-most
little moon is Amalthea. Below and to the right of Amalthea
are Metis and Adrastea. To the left of Adrastea is Thebe.
(Copyright Calvin J. Hamilton)

Nearly four centuries ago Galileo Galilei turned his homemade
telescope towards the heavens and discovered three points of light,
which at first he thought to be stars, hugging the planet Jupiter.
These stars were arranged in a straight
line with Jupiter. Sparking his interest, Galileo observed the
stars and found that they moved the wrong way. Four days later
another star appeared. After observing the stars over the next
few weeks, Galileo concluded that they were not stars but
planetary bodies in orbit around Jupiter.
These four stars have come to be know as the
Galilean satellites.

Over the course of the following centuries another 12 moons were
discovered bringing the total to 16.
During the last few years the total number of satellites that have discovered is 62.
Provisional designators are given until they are officially confirmed and named.
Finally in 1979, the
strangeness of these frozen new worlds was brought to light
by the Voyager spacecrafts as they swept past the Jovian system.
Again in 1996, the exploration of these worlds took a large
step forward as the Galileo spacecraft began its long term
mission of observing Jupiter and its moons.

Most of Jupiter's moons are relatively small and seem to
have been more likely captured
than to have been formed in orbit around Jupiter. The four large
Galilean moons,
Io,Europa,Ganymede and
Callisto,
are believed to have accreted
as part of the process by which Jupiter itself formed.
The following table summarizes the radius, mass, distance
from the planet center, discoverer and the date of discovery
of each of the moons of Jupiter: